(a) Field of the Invention
The invention concerns dense anodes of alloys of alkali metals, such as lithium for rechargeable batteries, preferably all solid. More specifically, the invention relates to discharged or partially discharged electrochemical rechargeable batteries as well as a method for the preparation of charged electrochemical rechargeable battery including a dense anode of an alloy of alkali metal, such as lithium.
(b) Description of Prior Art
The development of lithium rechargeable batteries which operate at low temperature is dependent on the design of lithium anodes which are sufficiently stable and reversible. With organic media (LiClO.sub.4 in propylene carbonate) or solid electrolytes (Li.sub.3 N or polyetherlithium salt complexes), the lithium electrode represents the component of a cell battery which fails most often because of the passivation of the electrode and the formation of surface dendrites. This phenomenon seriously limits the useful life of the battery.
Passivation is essentially due to a problem of thermodynamic instability of lithium with respect to some functional groups such as C--O, C.dbd.O, C--OH, etc . . . which provokes corrosion reactions with accumulation of reaction products at the interface. These products prevent any dense and uniform deposit of lithium. The dendrites appear during the recharge when a number of experimental parameters contribute to their formation: low temperature, high currents of recharge, passivated and rough surface, presence of asperities, etc . . . .
The use of lithium alloys whose chemical activity is lower than that of lithium, enables to eliminate most of the problems bound to the dendritic growth. Most of the alloys which are interesting from an electrochemical point of view (e.g. LiAl 50% at.) are fragile and brittle and thus are available in the form of powder. This lithium alloy powder can be used for the preparation of thin films of composite materials which are well suited for batteries operating with polymeric electrolytes, such as described in French Patent Applications of Michel Armand and Michel Duclot Nos. 78.32976, 78.32977 and 78.32978, now published under Nos. 2442512, 2442514 and 2442513, as well as U.S. Ser. No. 430,696 filed on Sept. 30, 1982 in the name of Hydro-Quebec.
Some alloys, such as the alloy of lithium and aluminum containing 90% at. lithium are sufficiently ductile to be laminated into thin films, but they are unacceptable because, on the one hand, their chemical potential is so close to lithium that they do not prevent dendritic growth, and on the other hand, irreversible phases of this composition are formed during cycling and are seriously harmful to the movement of lithium.
At temperatures higher than 80.degree. C. it has often been observed that composite anodes were aging prematurely during cycling. This phenomenon can be explained by a generalized reaction of the alloy of lithium with the surrounding electrolyte. To reduce the speed of the reaction one should conceive a way in which the alloy should contact the electrolyte only by one of its plane. This implies the use of thin, compact and uniform alloys of lithium. The thinness of the film of alloy results from the fact that at temperatures in the vicinity or lower than 100.degree. C. the coefficient of diffusion of the alkali metal in the alloy under consideration is often of the order of 10.sup.-9 cm.sup.2 sec.sup.-1 or lower. Therefore, to be able to utilize all the capacity of dense anodes, it is essential to rely on thin alloys. Unfortunately, these alloys are not presently commercially available.